Sense amplifiers are used in memory devices to read data from a memory. One example of a sense amplifier is a differential sense amplifier (also known as voltage mode sense amplifiers). A differential sense amplifier may be connected directly to bit lines, and may include a pair of transistors to read data from bit lines (complementary bit lines). The difference between bit line voltages determines the output of the amplifier. The performance of the differential sense amplifier may depend on the impedances of the bit lines. Ideally, the impedances of the pair of transistors should match to enable the differences between bit line voltages to be readily detected and amplified. Impedances for the pair of transistors are usually determined by resistive elements coupled to the transistors. Conventionally, these resistive elements may be Nwell resistors, poly resistors, and the like.
The conventional resistive elements are known to have some temperature dependence. The temperature dependence may cause the resistance values of the resistive elements to vary. Design considerations, such as, choosing appropriate sizes of resistors, are used to mitigate effects of temperature. However, choice of resistor sizes may not overcome variations in resistances that arise during manufacture. These variations in resistances result in mismatches at the inputs to the differential sense amplifier. The input mismatch results in an offset voltage in the sense amplifier. Other causes of offset voltages include unmatched parasitic capacitances and resistances at the inputs, or within the sense amplifier transistors, as well as mismatches in the differential pair transistor parameters caused by variations in the manufacturing process. The offset voltage introduces errors in read data. Consequently, there may be a loss of data due to the errors.
The present applicant has realized the need for an improved sense amplifier.